The present invention relates in general to the measurement of the partial pressure of a particular gas in a mixture of gases, and more particularly, to an instrument for measuring respiratory oxygen content which utilizes the paramagnetic properties of oxygen to generate an acoustic wave in the gas mixture by the rapid application of a magnetic field.
Techniques utilizing the paramagnetic properties of a gas, such as oxygen, to measure partial pressure of the gas in a mixture of gases are well known in the prior art. Oxygen is unique in that it is about 100 times more susceptible to magnetism than are the other respiratory gases. In the presence of a magnetic field, oxygen undergoes apparent changes in density, viscosity and thermal conductivity attributable to its paramagnetism. These changes may be modulated by changes in the temperature and magnetic field. All of these effects have been previously employed in instruments to measure the partial pressure of oxygen present in respiratory gases.
One type of apparatus is disclosed in U.S. Pat. No. 2,416,344 issued on Feb. 25, 1947, to Linus Pauling. Such apparatus is based on the principle that the force acting on a test body in an inhomogeneous magnetic field is dependent on the magnetic susceptibility of the gas surrounding the test body. Apparatus of the type described in U.S. Pat. No. 2,416,344 utilizes two glass spheres filled with nitrogen fixed at opposite ends of a horizontal bar suspended on quartz fiber placed in an inhomogeneous magnetic field. Any oxygen admitted to the apparatus will concentrate between the poles of the magnet and due to its relatively high density will displace the nitrogen filled spheres. The force displacing the spheres is proportional to the concentration of the oxygen present and is balanced by the suspension fiber torque. The angular deflection of the spheres is proportional to the concentration of oxygen present.
A paramagnetic gas analyzer of the "magnetic wind" type, which utilizes the principle that the magnetic susceptibility of oxygen varies inversely with its temperature, is disclosed by U.S. Pat. No. 3,616,679, issued to Emilio G. Meyer and Gianmario Bardoni on Nov. 2, 1971. The apparatus of U.S. Pat. No. 3,616,679 heats the sample of gas in an inhomogeneous magnetic field inside a cylindrical test chamber. When the sample gas includes a paramagnetic gas, such as oxygen, a magnetic wind will be generated in the direction of decreasing magnetic field intensity adjacent the heating elements. This wind flows over separate temperature responsive sensors, and the resulting change in temperature of the sensors is measured by a suitable electrical circuit. The output from the measurement circuit is a voltage proportional to the concentration of the paramagnetic gas present in the sample supplied to the apparatus.
U.S. Pat No. 4,173,975, issued to Daniel L. Delong and Edward L. Rich on Nov. 13, 1979, discloses an apparatus wherein a reference chamber is oscillated into and out of an air gap of a magnetic circuit by means of a piezoelectric bender bar. The magnetic circuit is immersed in an ambient gas mixture containing oxygen to be measured, and a background gas. This ambient gas fills the gap area resulting in a certain magnetic flux in the magnetic circuit. The movement of the reference chamber into and out of the gap will vary the magnetic flux, the amount of variation being indicative of the partial pressure of the oxygen present in the ambient gas mixture. A flux change sensing means is provided to measure the change in the magnetic flux in the magnetic circuit due to the reference chamber oscillating into and out of the gap.
U.S. Pat. No. 3,584,499, entitled Quick Response Oxygen Analyzer, issued to Heinz Hummel on June 15, 1971, discloses an apparatus which utilizes a pressure difference detector to measure the difference of alternating pressure of a sample gas and a comparison or reference gas in an alternating inhomogeneous magnetic field. The magnetic susceptibilities of the two gases depend proportionately on the oxygen content of the two gases. It can be demonstrated that the difference of alternating pressure is proportional to the susceptibility difference between the sample gas and the reference gas. Hence, if the oxygen content and the magnetic susceptibility of the reference gas is known, the oxygen content of the sample gas can be determined.
Apparatus measuring the oxygen uptake of a person breathing 100 percent oxygen on a breath by breath basis must meet exacting performance requirements. Normal oxygen uptake is on the order of several percent, and variations in the oxygen uptake of ten to twenty percent are significant. The mearsurement of oxygen uptake for a person receiving 100 percent oxygen therefore requires an accuracy on the order of 0.1 percent. Further, to make this measurement utilizing simultaneous measurement of flow and integration of the product of flow and oxygen concentration requires response time on the order of 0.050 seconds.
The thermomagnetic methods, as well as those dependent on diffusion about a heated filament, relying on sluggishness of molecules have a response time of about 0.1 second. Analyzers employing the Pauling principle cannot meet this quick response requirement because of high volume requirements and limited flow rates. Instruments utilizing thermomagnetic effects to cool a heated filament, magnetic wind, are influenced by the carrier gas and are quite delicate of construction and require much adjustment. Analyzers operating on the Pauling principle are also affected by the carrier gas. Further, almost all of the above-mentioned instruments require unusual types of magnetic pole pieces.